feat(stores)!: use backend storage references instead of configs (#3697)

**This PR changes configurations in a backward incompatible way.**

Run configs today repeat full SQLite/Postgres snippets everywhere a
store is needed, which means duplicated credentials, extra connection
pools, and lots of drift between files. This PR introduces named storage
backends so the stack and providers can share a single catalog and
reference those backends by name.

## Key Changes

- Add `storage.backends` to `StackRunConfig`, register each KV/SQL
backend once at startup, and validate that references point to the right
family.
- Move server stores under `storage.stores` with lightweight references
(backend + namespace/table) instead of full configs.
- Update every provider/config/doc to use the new reference style;
docs/codegen now surface the simplified YAML.

## Migration

Before:
```yaml
metadata_store:
  type: sqlite
  db_path: ~/.llama/distributions/foo/registry.db
inference_store:
  type: postgres
  host: ${env.POSTGRES_HOST}
  port: ${env.POSTGRES_PORT}
  db: ${env.POSTGRES_DB}
  user: ${env.POSTGRES_USER}
  password: ${env.POSTGRES_PASSWORD}
conversations_store:
  type: postgres
  host: ${env.POSTGRES_HOST}
  port: ${env.POSTGRES_PORT}
  db: ${env.POSTGRES_DB}
  user: ${env.POSTGRES_USER}
  password: ${env.POSTGRES_PASSWORD}
```

After:
```yaml
storage:
  backends:
    kv_default:
      type: kv_sqlite
      db_path: ~/.llama/distributions/foo/kvstore.db
    sql_default:
      type: sql_postgres
      host: ${env.POSTGRES_HOST}
      port: ${env.POSTGRES_PORT}
      db: ${env.POSTGRES_DB}
      user: ${env.POSTGRES_USER}
      password: ${env.POSTGRES_PASSWORD}
  stores:
    metadata:
      backend: kv_default
      namespace: registry
    inference:
      backend: sql_default
      table_name: inference_store
      max_write_queue_size: 10000
      num_writers: 4
    conversations:
      backend: sql_default
      table_name: openai_conversations
```

Provider configs follow the same pattern—for example, a Chroma vector
adapter switches from:

```yaml
providers:
  vector_io:
  - provider_id: chromadb
    provider_type: remote::chromadb
    config:
      url: ${env.CHROMADB_URL}
      kvstore:
        type: sqlite
        db_path: ~/.llama/distributions/foo/chroma.db
```

to:

```yaml
providers:
  vector_io:
  - provider_id: chromadb
    provider_type: remote::chromadb
    config:
      url: ${env.CHROMADB_URL}
      persistence:
        backend: kv_default
        namespace: vector_io::chroma_remote
```

Once the backends are declared, everything else just points at them, so
rotating credentials or swapping to Postgres happens in one place and
the stack reuses a single connection pool.

llama_stack/providers/utils/kvstore/kvstore.py

@@ -4,9 +4,17 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+from __future__ import annotations
+
+from llama_stack.core.storage.datatypes import KVStoreReference, StorageBackendConfig, StorageBackendType
 
 from .api import KVStore
-from .config import KVStoreConfig, KVStoreType
+from .config import KVStoreConfig
 
 
 def kvstore_dependencies():
@@ -44,20 +52,41 @@ class InmemoryKVStoreImpl(KVStore):
         del self._store[key]
 
 
-async def kvstore_impl(config: KVStoreConfig) -> KVStore:
-    if config.type == KVStoreType.redis.value:
+_KVSTORE_BACKENDS: dict[str, KVStoreConfig] = {}
+
+
+def register_kvstore_backends(backends: dict[str, StorageBackendConfig]) -> None:
+    """Register the set of available KV store backends for reference resolution."""
+    global _KVSTORE_BACKENDS
+
+    _KVSTORE_BACKENDS.clear()
+    for name, cfg in backends.items():
+        _KVSTORE_BACKENDS[name] = cfg
+
+
+async def kvstore_impl(reference: KVStoreReference) -> KVStore:
+    backend_name = reference.backend
+
+    backend_config = _KVSTORE_BACKENDS.get(backend_name)
+    if backend_config is None:
+        raise ValueError(f"Unknown KVStore backend '{backend_name}'. Registered backends: {sorted(_KVSTORE_BACKENDS)}")
+
+    config = backend_config.model_copy()
+    config.namespace = reference.namespace
+
+    if config.type == StorageBackendType.KV_REDIS.value:
         from .redis import RedisKVStoreImpl
 
         impl = RedisKVStoreImpl(config)
-    elif config.type == KVStoreType.sqlite.value:
+    elif config.type == StorageBackendType.KV_SQLITE.value:
         from .sqlite import SqliteKVStoreImpl
 
         impl = SqliteKVStoreImpl(config)
-    elif config.type == KVStoreType.postgres.value:
+    elif config.type == StorageBackendType.KV_POSTGRES.value:
         from .postgres import PostgresKVStoreImpl
 
         impl = PostgresKVStoreImpl(config)
-    elif config.type == KVStoreType.mongodb.value:
+    elif config.type == StorageBackendType.KV_MONGODB.value:
         from .mongodb import MongoDBKVStoreImpl
 
         impl = MongoDBKVStoreImpl(config)